In order to reduce the weight of the above-described compressor or the like, mostly both the sliding member and the opposite member are made of aluminum. In this case, since the aluminum members slide with one another, the following surface-treatments are applied on the vanes which are a sliding member.
According to one surface treatment, an electroless Ni--P plating or electroless Ni--B plating, which exhibits good wear resistance, is applied on the surface of the vanes. According to another surface treatment, hard additives, a solid lubricant and the like are dispersed in the electroless Ni--P plating coating to enhance the sliding characteristics. Enhancement of wear resistance is also achieved by heating the electroless Ni--P plating coating to 300.degree. C. or lower, and hence, hardening the coating.
The electroless Ni--P plating used for the surface treatment of the sliding member has usually P content of 6 to 10% and is, hence, of medium or high P type. The enhancement of sliding characteristics is attained by hardening, that is, increasing Hv 450-500 of hardness after the plating to extremely high hardness of from Hv 700-800 by means of heating to a temperature of 300.degree. C. or lower. For example, according to Japanese Unexamined Patent Publication (kokai) No. 64-32087, the electroless Ni--P plating coating with 8 to 10% of P is formed on vanes, and its hardness is increased to approximately Hv 700-800 by heat treatment. However, since the temperature of heat treatment for obtaining the coating hardening to as much as Hv 700-800 is high, the material strength of the aluminum alloy is lowered. The temperature of heat treatment is, therefore, limited. Actually, the hardness of coating attained is only from approximately Hv 500-600.
According to Japanese Unexamined Patent Publication (kokai) No. 8-158,058, there is proposed a sliding member, which is an aluminum-alloy substrate and a Ni--P--B based electroless plating which contains P--from 0.5 to 3.0 wt % and B--from 0.05 to 2.0 wt % and is applied on the substrate. A feature of the electroless plating coating is that it is hardened to an extremely high hardness of Hv 800 or more by heat treatment at a temperature which does not lead to appreciable reduction in the hardness of the aluminum alloy.
In addition, the electroless Ni--B plating used for the surface treatment of the sliding member has from 0.3 to 0.8% by weight of B content. Although the hardness of electroless Ni--B plating coating is enhanced to approximately Hv 800 by heat treatment, blister defects on the plating coating are liable to be caused by the heat treatment. Abnormal wear of the plating coating may, therefore, occur under particularly severe testing conditions.
Heretofore, the medium or high P type Ni--P plating coating has been applied on the surface of a sliding member, since high hardness is attained by heat treatment. The expected level of wear resistance of the NiP plating coating used, however, is not attained. Particularly, abnormal wear of the high-P type electroless plating coating occurred when the Al--Si alloy of a rotor or housing, i.e., the opposite material of the vanes, had coarse primary Si crystals of 50 .mu.m or more and/or numerous Si particle-number of more than 300/mm.sup.2. In addition, when the load was increased to augment the surface pressure of vanes, the abnormal wear occurred.
In addition, the known Ni--P--B electroless plating coating has usually lower P content than that of the known Ni--P electroless plating coating. However, since the crystallinity of Ni--P--B electroless plating coating is considerably disordered, problems in the sliding characteristics are incurred. It turned out that these problems are attributable to even a small amount of B disordering the crystallinity of Ni.
It is, therefore, an object of the present invention to apply a Ni-based electroless plating coating having excellent sliding characteristics on an aluminum-based sliding member, and to provide an aluminum-based sliding member which can be used under a high load.
It is another object of the present invention to provide a surface treating method which can enhance the sliding characteristics of the low-P type electroless Ni--P--B plating coating.